CHAPTER 8. The SCSI Interface

Transcription

1 CH08 8/1/01 2:34 PM Page 513 CHAPTER 8 The SCSI Interface

2 514 Chapter 8 The SCSI Interface Small Computer System Interface SCSI (pronounced scuzzy ) stands for Small Computer System Interface and is a general-purpose interface used for connecting many types of devices to a PC. This interface has its roots in SASI, the Shugart Associates System Interface. SCSI is the most popular interface for attaching high-speed disk drives to higher-performance PCs, such as workstations or network servers. SCSI is also very flexible; it is not only a disk interface, but is also a systems-level interface allowing many types of devices to be connected. SCSI is a bus that supports as many as 7 or 15 total devices. Multichannel adapters exist that can support up to 7 or 15 devices per channel. The SCSI controller, called the host adapter, functions as the gateway between the SCSI bus and the PC system bus. Each device on the bus has a controller built in. The SCSI bus does not talk directly with devices such as hard disks; instead, it talks to the controller that is built into the drive. A single SCSI bus can support as many as 8 or 16 physical units, usually called SCSI IDs. One of these units is the SCSI host adapter card in your PC; the other 7 or 15 can be other peripherals. You could have hard disks, tape drives, CD-ROM drives, a graphics scanner, or other devices attached to a single SCSI host adapter. Most systems can support up to four host adapters, each with up to 15 devices, for a total of 60 devices! There are even dual channel adapters that could double that figure. SCSI is a fast interface, generally suited to high-performance workstations, servers, or anywhere the ultimate in performance for a storage system interface is needed. The latest Ultra4 (Ultra320) SCSI version supports transfer speeds of up to 320MB/sec! When you purchase a SCSI device such as a SCSI hard disk, you usually are purchasing the device, controller, and SCSI adapter in one circuit; as such the device is ready to connect directly to the SCSI bus. This type of drive usually is called an embedded SCSI device the SCSI interface is built in. For example, most SCSI hard drives are technically the same as their IDE counterparts except for the addition of the SCSI bus adapter circuits (normally a single chip) added to the controller board. You do not need to know what type of controller is inside the SCSI drive because your system cannot talk directly to the controller as though it were plugged into the system bus, like on a standard IDE drive. Instead, communications go through the SCSI host adapter installed in the system bus. You can access the drive only with the SCSI protocols. Apple originally rallied around SCSI as being an inexpensive way out of the bind in which it put itself with the Macintosh. When the engineers at Apple realized their mistake in making the Macintosh a closed system (with no expansion slots), they decided that the easiest way to gain expandability was to build a SCSI port into the system, which is how external peripherals were originally added to the slotless Macs. Of course, in keeping with Apple tradition, they used a nonstandard SCSI connector. Now that Apple is designing systems with expansion slots, Universal Serial Bus (USB), and FireWire (ilink or IEEE-1394), SCSI has been dropped from most Macs as a built-in option. Because PC systems always have been expandable, the push toward SCSI has not been as urgent. With up to eight or more bus slots supporting various devices and controllers in PC-compatible systems, it seemed as though SCSI was not as necessary for system expansion. In fact, with modern PCs sporting inexpensive builtin USB ports for external expansion, in most cases SCSI devices are necessary only when top performance is a critical issue. SCSI has become popular in the PC-based workstation market because of the performance and expandability it offers. One block that stalled acceptance of SCSI in the early PC marketplace was the lack of a real standard; the SCSI standard originally was designed by one company and then turned into a committee-controlled public standard. Since then, no single manufacturer has controlled it.

3 ANSI SCSI Standards Chapter Note Most SCSI host adapters bundled with hardware, such as graphics scanners or SCSI CD-ROM, CD-R, or CD-RW drives, will not include all the features needed to support multiple SCSI devices or bootable SCSI hard drives. This has nothing to do with any limitations in the SCSI specification. The situation is simply that the manufacturer has included the most stripped-down version of a SCSI adapter available to save money. It has all the functionality necessary to support the device it came with, but nothing else. Fortunately, with the right adapter and drivers, one SCSI card could support all the SCSI devices in a system from hard drives to optical drives, scanners, tape drives, and more. In the beginning, SCSI adapters lacked the capability to boot from hard disks on the SCSI bus. Booting from these drives and using a variety of operating systems was a problem that resulted from the lack of a software interface standard. The standard BIOS software in PC systems is designed to talk to ST-506/412, ESDI, or ATA (IDE) hard disks and devices. SCSI is so different from ATA/IDE that a new set of ROM BIOS routines is necessary to support the system so it can self-boot. Also, this BIOS support is unique to the SCSI host adapter you are using; so, unless the host adapter is built into your motherboard, this support won t be found in your motherboard BIOS. Instead, SCSI host adapters are available with BIOS support for SCSI hard disk drives right on the SCSI host adapter itself. Note For more information about the ST-506/412 Interface and the ESDI Interface, see ST-506/412 Interface and ESDI Interface, respectively, in the Technical Reference section of the CD accompanying this book. An expanded discussion of both technologies can be found in Upgrading and Repairing PCs, 6th Edition, which is included in its entirety in PDF format on the CD with this book. Because of the lead taken by Apple in developing systems software (operating systems and ROM) support for SCSI, peripherals connect to Apple systems in fairly standard ways. Until recently, this kind of standard-setting leadership was lacking for SCSI in the PC world. This situation changed dramatically with Windows 95 and later versions, which include drivers for most popular SCSI adapters and peripherals on the market. These days, Windows 98/Me and Windows 2000 include even more drivers and support for SCSI adapters and devices built in. Many PC manufacturers have standardized SCSI for high-end systems. In these systems, a SCSI host adapter card is placed in one of the slots, or the system has a SCSI host adapter built into the motherboard. This arrangement is similar in appearance to the IDE interface because a single cable runs from the motherboard to the SCSI drive. SCSI supports as many as 7 or 15 additional devices per bus (some of which might not be hard disks), whereas IDE supports only 4 devices (2 per controller). Additionally, SCSI supports more types of devices other than hard disks than IDE supports. IDE devices must be a hard disk, an IDE-type CD-ROM drive, a tape drive, an LS-120 SuperDisk drive, a Zip drive, and so on. Systems with SCSI drives are easy to upgrade because virtually any third-party SCSI drive will plug in and function. ANSI SCSI Standards The SCSI standard defines the physical and electrical parameters of a parallel I/O bus used to connect computers and peripheral devices in daisy-chain fashion. The standard supports devices such as disk drives, tape drives, and CD-ROM drives. The original SCSI standard (ANSI X ) was approved in 1986, SCSI-2 was approved in January 1994, and the first portions of SCSI-3 were approved in Note that SCSI-3 has evolved into an enormous standard with numerous sections and is an evolving, growing standard still very much under development. Because it has been broken down into multiple standards, there really is no single SCSI-3 standard.

4 516 Chapter 8 The SCSI Interface The SCSI interface is defined as a standard by ANSI (American National Standards Institute), specifically by a committee currently known as T10. T10 is a technical committee of the National Committee on Information Technology Standards (NCITS, pronounced insights ). NCITS is accredited by ANSI and operates under rules approved by ANSI. These rules are designed to ensure that voluntary standards are developed by the consensus of industry groups. NCITS develops informationprocessing system standards, whereas ANSI approves the process under which they are developed and publishes them. Working draft copies of all SCSI-related standards can be downloaded from the T10 Technical Committee site at One problem with the original SCSI-1 document was that many of the commands and features were optional, and there was little or no guarantee that a particular peripheral would support the expected commands. This problem caused the industry as a whole to define a set of 18 basic SCSI commands called the Common Command Set (CCS) to become the minimum set of commands supported by all peripherals. CCS became the basis for what is now the SCSI-2 specification. Along with formal support for CCS, SCSI-2 provided additional definitions for commands to access CD-ROM drives (and their sound capabilities), tape drives, removable drives, optical drives, and several other peripherals. In addition, an optional higher speed called Fast SCSI-2 and a 16-bit version called Wide SCSI-2 were defined. Another feature of SCSI-2 is command queuing, which enables a device to accept multiple commands and execute them in the order that the device deems to be most efficient. This feature is most beneficial when you are using a multitasking operating system that could be sending several requests on the SCSI bus at the same time. The X3T9 group approved the SCSI-2 standard as X in August 1990, but the document was recalled in December 1990 for changes before final ANSI publication. Final approval for the SCSI-2 document was finally made in January 1994, although it has changed little from the original 1990 release. The SCSI-2 document is now called ANSI X The official document is available from Global Engineering Documents or the ANSI committee both are listed in the Vendor List on the CD. You can also download working drafts of these documents from the T10 Technical Committee home page as listed previously. Most companies indicate that their host adapters follow both the ANSI X (SCSI-1) and the X (SCSI-2) standards. Note that because virtually all parts of SCSI-1 are supported in SCSI- 2, virtually any SCSI-1 device is also considered SCSI-2 by default. Many manufacturers advertise that their devices are SCSI-2, but this does not mean they support any of the additional optional features that were incorporated in the SCSI-2 revision. For example, an optional part of the SCSI-2 specification includes a fast synchronous mode that doubles the standard synchronous transfer rate from 5MB/sec to 10MB/sec. This Fast SCSI transfer mode can be combined with 16-bit Wide SCSI for transfer rates of up to 20MB/sec. An optional 32-bit version was defined in SCSI-2, but component manufacturers have shunned this as too expensive. In essence, 32-bit SCSI was a stillborn specification, as it was withdrawn from the SCSI-3 standard. Most SCSI implementations are 8-bit standard SCSI or 16-bit Fast/Wide SCSI. Even devices that support none of the Fast or Wide modes can still be considered SCSI-2. SCSI-3 is broken down into a number of standards. The SCSI Parallel Interface (SPI) standard controls the parallel interconnection between SCSI devices, which is mostly what we are talking about here. So far several versions of SPI have existed, including SPI, SPI-2, SPI-3, and SPI-4. Versions through SPI-3 have been published, whereas SPI-4 is still in draft form. What can be confusing is that several terms can be used to describe the newer SPI standards, as shown in Table 8.1.

5 SCSI-1 Chapter Table 8.1 SPI (SCSI Parallel Interface) Standards SCSI-3 Standard Also Known As Speed Throughput SPI Ultra SCSI Fast-20 20/40MB/sec SPI-2 Ultra2 SCSI Fast-40 40/80MB/sec SPI-3 Ultra3 SCSI Fast-80DT 160MB/sec SPI-4 Ultra4 SCSI Fast-160DT 320MB/sec To add to the confusion, SPI-3 or Ultra3 SCSI is also called Ultra160 or Ultra160+, and SPI-4 or Ultra4 SCSI is also called Ultra320 or Ultra320+ by some companies. The Ultra160/320 designation refers to any device that includes the first three of the five main features from the Ultra3/4 SCSI specification. Ultra160/320+ refers to any device that supports all five main features of Ultra3/4 SCSI. Table 8.2 shows the maximum transfer rates for the SCSI bus at various speeds and widths and the cable type required for the specific transfer widths. Note The A cable is the standard 50-pin SCSI cable, whereas the P cable is a 68-pin cable designed for 16-bit transfers. High Voltage Differential (HVD) signaling was never popular and is now considered obsolete. LVD (Low Voltage Differential) signaling is used in the Ultra2 and Ultra3 modes to increase performance and cabling lengths. Pinouts for the cable connections are listed in this chapter in Tables SCSI is both forward and backward compatible, meaning one can run faster devices on buses with slower host adapters or vice versa. In each case, the entire bus will run at the lowest common denominator speed. In fact, as was stated earlier, virtually any SCSI-1 device can also legitimately be called SCSI-2 (or even SCSI-3) because most of the improvements in the later versions are optional. Of course, you can t take advantage of the faster modes on an older, slower host adapter. By the same token, you can purchase an Ultra3 capable SCSI host adapter and still run older standard SCSI devices. You can even mix standard 8-bit and wide 16-bit devices on the same bus using cable adapters. SCSI-1 SCSI-1 was the first implementation of SCSI. It was officially known as ANSI X The major features of SCSI-1 were 8-bit parallel bus 5MHz asynchronous or synchronous operation 4MB/sec (asynchronous) or 5MB/sec (synchronous) throughput 50-pin cables with low-density pin-header internal and Centronics-style external connectors Single-ended (SE) unbalanced transmission Passive termination Optional bus parity SCSI-1 is now considered obsolete; in fact, the standard has been withdrawn by ANSI and replaced by SCSI-2.

6 518 Chapter 8 The SCSI Interface Table 8.2 SCSI Types, Data-Transfer Rates, and Cables Clock SCSI SCSI Marketing Speed Transfer Standard Technology Term (MHz) Width SCSI-1 Async Asynchronous 5 8-bit SCSI-1 Fast-5 Synchronous 5 8-bit SCSI-2 Fast-5/Wide Wide 5 16-bit SCSI-2 Fast-10 Fast 10 8-bit SCSI-2 Fast-10/Wide Fast/Wide bit SPI (SCSI-3) Fast-20 Ultra 20 8-bit SPI (SCSI-3) Fast-20/Wide Ultra/Wide bit SPI-2 (SCSI-3) Fast-40 Ultra bit SPI-2 (SCSI-3) Fast-40/Wide Ultra2/Wide bit SPI-3 (SCSI-3) Fast-80DT Ultra3 (Ultra160) bit SPI-4 (SCSI-3) Fast-160DT Ultra4 (Ultra320) bit *Not including the host adapter. Cable Lengths are in meters: 25M = 80ft., 12M = 40ft., 6M = 20ft., 3M = 10ft., 1.5M = 5ft. SE = Single-ended signaling; HVD = High Voltage Differential signaling, obsolete LVD = Low Voltage Differential signaling SPI = SCSI Parallel Interface, part of SCSI-3 SCSI-2 SCSI-2 is officially known as ANSI X The SCSI-2 specification is essentially an improved version of SCSI-1 with some parts of the specification tightened and several new features and options added. Normally, SCSI-1 and SCSI-2 devices are compatible, but SCSI-1 devices ignore the additional features in SCSI-2. Some of the changes in SCSI-2 are very minor. For example, SCSI-1 allowed SCSI bus parity to be optional, whereas parity must be implemented in SCSI-2. Parity is an extra bit that is sent as a verification bit to ensure that the data is not corrupted. Another requirement is that initiator devices, such as host adapters, provide terminator power to the interface; most devices already did so. SCSI-2 also added several optional features: Fast SCSI (10MHz) Wide SCSI (16-bit transfers) Command queuing New commands High-density, 50-pin cable connectors Active (Alternative 2) termination for improved single-ended (SE) transmission High Voltage Differential (HVD) transmission (incompatible with SE on the same bus) for longer bus lengths

7 SCSI-2 Chapter Transfer Max. Max. Max. Max. Speed No. of Length Length Length (MB/s) Devices* Cable Type (SE) (HVD) (LVD) 4 7 A (50-pin) 6M 25M A (50-pin) 6M 25M P (68-pin) 6M 25M A (50-pin) 3M 25M P (68-pin) 3M 25M A (50-pin) 3/1.5M 1 25M P (68-pin) 3/1.5M 1 25M A (50-pin) M P (68-pin) M P (68-pin) M P (68-pin) M 2 DT = Double transition, or two transfers per clock cycle, 16-bit only 1 = Ultra SCSI cable total length is restricted to 1.5M if more than 3 devices exist on the bus (not including the host adapter). A maximum of 7 devices is allowed. 2 = A 25M cable may be used if only one device exists (point-to-point interconnect). 3 = Ultra3 (Ultra160) and Ultra4 (Ultra320) SCSI transfer twice per clock cycle and are 16-bit only. Wide SCSI enables parallel data transfer at a bus width of 16 bits. The wider connection requires a new cable design. The standard 50-conductor, 8-bit cable is called the A cable. SCSI-2 originally defined a special 68-conductor B cable that was supposed to be used in conjunction with the A cable for 32-bit wide transfers. However, because of a lack of industry support and the added expenses involved, 32-bit SCSI was never actually implemented and was finally removed as a part of the SCSI-3 specifications. Therefore, two different types of SCSI cables are now available, called the A cable and the P cable. A cables are any SCSI cables with 50-pin connectors, whereas P cables are any SCSI cables with 68-pin connectors. You need a P cable if you are connecting a Wide SCSI device and want it to work in 16-bit mode. The P cable was not officially included in the standard until SCSI-3. Fast SCSI refers to high-speed synchronous transfer capability. Fast SCSI achieves a 10MB/sec transfer rate on the standard 8-bit SCSI cabling. When combined with a 16-bit Wide SCSI interface, this configuration results in data-transfer rates of 20MB/sec (called Fast/Wide). The high-density connectors enable smaller, more efficient connector and cable designs. In SCSI-1, an initiator device, such as a host adapter, was limited to sending one command per device. In SCSI-2, the host adapter can send as many as 256 commands to a single device, which will store and process those commands internally before responding on the SCSI bus. The target device even can resequence the commands to allow for the most efficient execution or performance possible. This is especially useful in multitasking environments, such as OS/2 and Windows NT, which can take advantage of this feature. SCSI-2 took the Common Command Set that was being used throughout the industry and made it an official part of the standard. The CCS was designed mainly for disk drives and did not include specific

8 520 Chapter 8 The SCSI Interface commands designed for other types of devices. In SCSI-2, many of the old commands are reworked, and several new commands have been added. New command sets have been added for CD-ROMs, optical drives, scanners, communications devices, and media changers (jukeboxes). The single-ended SCSI bus depends on very tight termination tolerances to function reliably. Unfortunately, the original 132-ohm passive termination defined in the SCSI-1 document was not designed for use at the higher synchronous speeds now possible. These passive terminators can cause signal reflections to generate errors when transfer rates increase or when more devices are added to the bus. SCSI-2 defines an active (voltage-regulated) terminator that lowers termination impedance to 110 ohms and improves system integrity. Note that LVD SCSI requires special LVD terminators. If you use SE terminators on a bus with LVD devices, they either won t work or, if they are multimode devices, will default to SE operation. These features are not required; they are optional under the SCSI-2 specification. If you connect a standard SCSI host adapter to a Fast SCSI drive, for example, the interface will work, but only at standard SCSI speeds. SCSI-3 SCSI-3 is a term used to describe a set of standards currently being developed. Simply put, it is the next generation of documents a product conforms to. Unlike SCSI-1 and SCSI-2, SCSI-3 is not one document that covers all the layers and interfaces of SCSI, but is instead a collection of documents that covers the primary commands, specific command sets, and electrical interfaces and protocols. The command sets include hard disk interface commands, commands for tape drives, controller commands for RAID (Redundant Array of Inexpensive Drives), and other commands as well. There is also an overall SCSI Architectural Model (SAM) for the physical and electrical interfaces, as well as a SCSI Parallel Interface standard that controls the form of SCSI most commonly used. Each document within the standard is now a separate publication with its own revision level for example, within SCSI-3 three different versions of the SCSI Parallel Interface have been published. Normally we don t refer to SCSI-3 anymore as a specific interface and instead refer to the specific subsets of SCSI-3, such as SPI-3 (Ultra3 SCSI). The main additions to SCSI-3 include Ultra2 (Fast-40) SCSI Ultra3 (Fast-80DT) SCSI Ultra4 (Fast-160DT) SCSI New Low Voltage Differential signaling Elimination of High Voltage Differential signaling Breaking up SCSI-3 into many smaller individual standards has enabled the standard as a whole to develop more quickly. The individual substandards can now be published rather than waiting for the entire standard to be approved. Figure 8.1 shows the main parts of SCSI-3. The primary changes being seen in the marketplace from SCSI-3 are the new Fast-40 (Ultra2) and Fast- 80DT (Ultra3) high-speed drives and adapters. These have taken the performance of SCSI up to 160MB/sec. Also new is the LVD electrical standard, which allows for greater cable lengths. The older High Voltage Differential signaling has been removed from the standard. A number of people are confused over the speed variations in SCSI. Part of the problem is that speeds are quoted as either clock speeds (MHz) or transfer speeds. With 8-bit transfers, you get one byte per transfer, so if the clock is 40MHz (Fast-40 or Ultra2 SCSI), the transfer speed is 40MB/sec. On the

9 SCSI-3 Chapter other hand, if you are using a Wide (16-bit) interface, the transfer speed doubles to 80MB/sec, even though the clock speed remains at 40MHz. With Fast-80DT, the bus speed technically remains at 40MHz; however, two transfers are made per cycle, resulting in a throughput speed of 160MB/sec. The same is true for Ultra4 SCSI, which runs at 80MHz and transfers 2 bytes at a time and two transfers per cycle. Ultra4 is also called Ultra320 and is the fastest form of parallel SCSI today. Common Access Method {CAM-3} Block Commands {SBC} Reduced Block Commands {RBC} Stream Commands {SSC} Medium Changer Commands {SMC} Multimedia Commands {MMC, MMC-2} Controller Commands {SSC, SSC-2} Enclosure Services {SES} Primary Commands {SPC, SPC-2} Architectural Model {SAM, SAM-2} Interlocked Protocol {SIP} Parallel Interface {SPI} Fast-20 {aka, Ultra} Parallel Interface-2 {SPI-2} [will replace SIP, SPI, and Fast-20] {aka, Ultra2} Parallel Interface-3 {SPI-3} [new project based on SPI-2] {aka, Ultra3} Serial Bus Protocol-2 {SBP-2} IEEE 1394 Fiber Channel Protocol {FCP, FCP-2} Fiber Channel SSA SCSI-3 Protocol {SSA-S3P} SSA-TC2 SSA-PH1 or SSA-PH2 Figure 8.1 The SCSI-3 architecture. Finally, confusion exists because SCSI speeds or modes are often discussed using either the official terms, such as Fast-10, Fast-20, Fast-40, and Fast-80DT, or the equivalent marketing terms, such as Fast, Ultra, Ultra2, and Ultra3 (also called Ultra160). Refer to Table 8.2 for a complete breakdown of SCSI official terms, marketing terms, and speeds. The further evolution of the most commonly used form of SCSI is defined under the SPI standards within SCSI-3. The SPI standards are detailed in the following sections. SPI or Ultra SCSI The SCSI Parallel Interface standard was the first SCSI standard that fell under the SCSI-3 designation and is officially known as ANSI X SPI is also called Ultra SCSI by most marketing departments and defines the parallel bus electrical connections and signals. A separate document called the SCSI Interlock Protocol (SIP) defines the parallel command set. SIP was included in the later SPI-2 and SPI-3 revisions and is no longer carried as a separate document. The main features added in SPI or Ultra SCSI are Fast-20 (Ultra) speeds (20MB/sec or 40MB/sec) 68-pin P-cable and connectors defined for Wide SCSI

10 522 Chapter 8 The SCSI Interface SPI initially included speeds up to Fast SCSI (10MHz), which enables transfer speeds up to 20MB/sec using a 16-bit wide bus. Later, Fast-20 (20MHz), commonly known as Ultra SCSI, was added through an addendum document (ANSI X ), allowing a throughput of 40MB/sec on a 16-bit wide bus (commonly called Ultra/Wide). SPI-2 or Ultra2 SCSI SPI-2 is also called Ultra2 SCSI, officially published as ANSI X , and adds several features to the prior versions: Fast-40 (Ultra2) speeds (40MB/sec or 80MB/sec) Low Voltage Differential signaling Single Connector Attachment (SCA-2) connectors 68-pin Very High Density Connector (VHDC) The most notable of these is a higher speed called Fast-40, which is commonly called Ultra2 SCSI and runs at 40MHz. On a narrow (8-bit) bus, this results in 40MB/sec throughput, whereas on a wide bus (16-bit), this results in 80MB/sec throughput and is commonly referred to as Ultra2/Wide. To achieve these speeds, a new electrical interface called LVD must be used. The slower single-ended electrical interface is only good for speeds up to Fast-20. Fast-40 mode requires LVD operation. The LVD signaling also enables longer cable lengths up to 12 meters with multiple devices or 25 meters with only one device. LVD and SE devices can share the same cable, but in that case the bus will run in SE mode and be restricted in length to as little as 1.5 meters in Fast-20 mode. LVD operation requires special LVD-only or LVD/SE multimode terminators. If multimode terminators are used, the same terminators will work on either SE or LVD buses. The SPI-2 standard also includes SIP (SCSI Interlink Protocol) and defines the Single Connector Attachment (SCA-2) 80-pin connector for hot-swappable drive arrays. There is also a new 68-pin Very High Density Connector (VHDC), which is smaller than the previous types. SCSI Signaling Normal, or standard, SCSI uses a signaling technique called single-ended signaling. SE signaling is a low-cost technique, but it also has performance and noise problems. Single-ended signaling is also called unbalanced signaling. Each signal is carried on a pair of wires, normally twisted to help reduce noise. With SE one of the pair is grounded, often to a common ground for all signals, and the other carries the actual voltage transitions. It is up to a receiver at the other end of the cable to detect the voltage transitions, which are really just changes in voltage. Unfortunately, this type of unbalanced signaling is very prone to problems with noise, electromagnetic interference, and ground leakage; these problems get worse the longer the cable is. This is the reason Ultra SCSI was limited to such short maximum bus lengths as little as 1 1/2 meters or 5 feet. When SCSI was first developed, a signaling technique called High Voltage Differential signaling was also introduced into the standard. Differential signaling, also known as balanced signaling, is still done with a pair of wires. In fact, the first in the pair carries the same type of signal that single-ended SCSI carries. The second in the pair, however, carries the logical inversion of that signal. The receiving device detects the difference between the pair (hence the name differential). By using the wires in a balanced pair, the receiver no longer needs to detect voltage magnitude, only the differential between voltage in two wires. This is much easier for circuits to do reliably, which makes them less susceptible to noise and enables greater cable length. Because of this, differential SCSI can be used with cable lengths of up to 25 meters, whereas single-ended SCSI is good only for 6 meters maximum, or as little as 1 1/2 meters in the faster modes.

11 SCSI-3 Chapter Figure 8.2 shows the circuit differences between balanced (differential) and unbalanced (single-ended) transmission lines. - signal + signal Balanced Figure 8.2 Unbalanced Balanced (differential) versus unbalanced (single-ended) signaling. Unfortunately, the original standard for HVD signaling called for high voltage differentials between the two wires. This means that small, low-power, single-chip interfaces using HVD signaling could not be developed. Instead, circuits using several chips were required. This works at both ends, meaning both the host adapter and device circuitry had to be larger and more expensive. Another problem with HVD SCSI is that although the cables and connectors look (and are) exactly the same as for SE SCSI, both types of devices cannot be mixed on the same bus. If they are, the high voltage from the HVD device will burn out the receiver circuits on all SE devices attached to the bus. In other words, the result will be smoked hardware not a pretty sight. Because SE SCSI worked well enough for the speeds that were necessary up until recently, HVD SCSI signaling never really caught on. It was used only in minicomputers and very rarely, if at all, in PCs. Because of this, the extra cost of this interface, and the fact that it is electrically incompatible with standard SE SCSI devices, HVD signaling was removed from the SCSI specification in the latest SCSI-3 documents. So, as far as we are concerned, it is obsolete. Still, a need existed for a more reliable signaling technique that would allow for longer cable lengths. The answer came in the form of LVD signaling. By designing a new version of the differential interface, it can be made to work with inexpensive and low-power SCSI chips. Another advantage of LVD is that because it uses low voltage, if you plug an LVD device into an SE SCSI bus, nothing will be damaged. In fact, as an optional part of the LVD standard, the LVD device can be designed as a multimode device, which means it works on both SE and LVD buses. In the case of installing a multimode LVD device into an SE bus, the device detects that it is installed in an SE bus and defaults to SE mode. This means that all multimode LVD/SE SCSI devices can be used on either LVD or SE SCSI buses. However, when on a bus with even one other SE device, all the LVD devices on the bus run only in SE mode. Because SE mode supports only SCSI speeds of up to 20MHz (Fast-20 or UltraSCSI) and cable lengths of up to 1 1/2 or 3 meters, the devices also work only at that speed or lower; you also might have problems with longer cables. Although you can purchase an Ultra3 SCSI multimode LVD/SE drive and install it on a SCSI bus along with single-ended devices, you will certainly be wasting the capabilities of the faster device. Note that all Ultra2 and Ultra3 devices support LVD signaling because that is the only way they can be run at the Ultra2 (40MHz) or Ultra3 (80MHz) speeds. Ultra SCSI (20MHz) or slower devices can support LVD signaling, but in most cases LVD is synonymous with Ultra2 or Ultra3 only. Table 8.2, shown earlier, lists all the SCSI speeds and maximum lengths for each speed using the supported signaling techniques for that speed.

12 524 Chapter 8 The SCSI Interface Because the connectors are the same for SE, HVD, LVD, or multimode SE/LVD devices, and because putting an HVD device on any bus with SE or LVD devices causes damage, it would be nice to be able to tell them apart. One way is to look for a special symbol on the unit; the industry has adopted different universal symbols for single-ended and differential SCSI. Figure 8.3 shows these symbols. SCSI SE SCSI LVD Single-Ended SCSI Devices Low Voltage Differential SCSI SCSI LVD/SE SCSI DIFF Low Voltage Differential/Single-Ended Multi-mode SCSI High Voltage Differential SCSI Figure 8.3 Universal symbol icons identifying SE, LVD, multimode LVD/SE, and HVD devices. If you do not see such symbols, you can tell whether you have a High Voltage Differential device by using an ohmmeter to check the resistance between pins 21 and 22 on the device connector: On a single-ended or Low Voltage Differential device, the pins should be tied together and also tied to the ground. On a High Voltage Differential device, the pins should be open or have significant resistance between them. Although you will blow up stuff if you plug HVD devices into LVD or SE buses, this generally should not be a problem because virtually all devices used in the PC environment are SE, LVD, or LVD/SE. HVD has essentially been rendered obsolete because it has been removed from the SCSI standard with Ultra3 SCSI (SPI-3). SPI-3 or Ultra3 SCSI (Ultra160) SPI-3, also known as Ultra3 or Ultra160 SCSI, builds on the previous standard and doubles the speed again to Fast-80DT (double transition). This results in a maximum throughput of 160MB/sec. The main features added to SPI-3 (Ultra3) are DT (double transition) clocking CRC (Cyclic Redundancy Check) Domain validation Packetization Quick Arbitrate and Select (QAS) Double transition clocking sends data on both the rising and falling edges of the REQ/ACK clock. This enables Ultra3 SCSI to transfer data at 160MB/sec, while still running at a bus clock rate of 40MHz. This mode is defined for 16-bit wide bus use only.

13 SCSI-3 Chapter Cyclic Redundancy Checking (CRC) is a form of error checking incorporated into Ultra3 SCSI. Previous versions of SCSI used simple parity checking to detect transmission errors. CRC is a much more robust form of error-detection capability that is far superior for operation at higher speeds. Domain validation allows better negotiation of SCSI transfer speeds and modes. With prior SCSI versions, when the bus is initialized, the host adapter sends an INQUIRY command at the lowest 5MHz speed to each device to determine which data-transfer rate the device can use. The problem is that even though both the host adapter and device might support a given speed, there is no guarantee that the interconnection between the devices will reliably work at that speed. If a problem occurs, the device becomes inaccessible. With domain validation, after a maximum transfer speed is negotiated between the host and the device, it is then tested at that rate. If errors are detected, the rate is stepped down until the connection tests error-free. This is similar to how modems negotiate transmission speeds before communicating and will go a long way toward improve the flexibility and perceived reliability of SCSI. Packetization is a protocol that enables information to be transferred between SCSI devices in a much more efficient manner. Traditional parallel SCSI uses multiple bus phases to communicate different types of information between SCSI devices: one for command information, two for messages, one for status, and two for data. In contrast, packetized SCSI communicates all this information by using only two phases: one for each direction. This dramatically reduces the command and protocol overhead, especially as higher and higher speeds are used. Packetized SCSI is fully compatible with traditional parallel SCSI, which means packetized SCSI devices can reside on the same bus as traditional SCSI devices. As long as the host adapter supports the packetization, it can communicate with one device using packets and another using traditional protocol. Not all Ultra3 or Ultra160 SCSI devices include packetization support. Ultra3 devices that support packetization normally are referred to as Ultra160+ SCSI. Quick Arbitrate and Select (QAS) is a feature in Ultra3 SCSI that reduces arbitration time by eliminating bus free time. QAS enables a device to transfer control of the bus to another device without an intervening BUS FREE phase. SCSI devices that support QAS report that capability in the INQUIRY command. Ultra160 and Ultra160+ Because the five main new features of Ultra3 SCSI are optional, drives could claim Ultra3 capability and not have a consistent level of functionality. To ensure truth in advertising and a minimum level of performance, a group of manufacturers got together and created a substandard within Ultra3 SCSI that requires a minimum set of features. These are called Ultra160 and Ultra160+ because both indicate 160MB/sec throughput. These new substandards are not an official part of SCSI they are not an official part of the standard. Even so, they do guarantee that certain specifications will be met and certain performance levels will be attained. Ultra160 is a specific implementation of Ultra3 (SPI-3) SCSI that includes the first three additional features of Ultra3 SCSI: Fast-80DT clocking for 160MB/sec operation CRC Domain validation Ultra160 SCSI runs in LVD mode and is backward compatible with all Ultra2 SCSI (LVD) devices. The only caveat is that no SE devices must be on the bus. When Ultra2 and Ultra160 (Ultra3) devices are mixed, each device can operate at its full-rated speed independent of the other. The bus will dynamically switch from single- to double-transition mode to support the differences in speeds.

14 526 Chapter 8 The SCSI Interface Ultra160+ adds the other two features, ensuring a full implementation of Ultra3: Packetization Quick Arbitrate and Select With Ultra160 and Ultra160+, you have a known level of functionality to ensure that a minimum level of performance will be met. Ultra160+ SCSI is the highest-performance PC-level storage interface and is best suited for high-traffic environments, such as high-end network servers or workstations. The adaptability and scalability of the interface enables high performance with high reliability. SPI-4 or Ultra4 SCSI (Ultra320) SPI-4, also known as Ultra4 or Ultra320 SCSI, is basically an update on the previous Ultra3 (Ultra160) SCSI. It has all the same features, except it doubles the speed again to Fast-160DT. This results in a maximum throughput of 320MB/sec, the current fastest form of parallel SCSI. The SPI-4 standard is still in development, although products running at Ultra320 speeds will likely be available before the standard is officially published. Fiber Channel SCSI Fiber Channel SCSI is a specification for a serial interface using a fiber channel physical and protocol characteristic, with a SCSI command set. It can achieve 100MB/sec over either fiber or coaxial cable of several kilometers in length. Fiber Channel is designed for long-distance connectivity (such as several kilometers) and connecting multiple systems. Standard parallel SCSI will continue to be the I/O choice for inside the box or external close proximity connectivity for some time to come. Due to compatibility problems between various manufacturers Fiber Channel devices and the fact that Ultra3 (Ultra160) and Ultra4 (Ultra320) SCSI are significantly faster, Fiber Channel is unlikely to become popular in the PC environment. Ultra160/320 SCSI is also far less expensive to implement and remains backward compatible with Ultra2 SCSI devices. SCSI Cables and Connectors The SCSI standards are very specific when it comes to cables and connectors. The most common connectors specified in this standard are the 50-position unshielded pin header connector for internal SCSI connections and the 50-position shielded Centronics latch-style connectors for external connections. The shielded Centronics-style connector also is called Alternative 2 in the official specification. Passive or Active termination (Active is preferred) is specified for both single-ended and differential buses. The 50-conductor bus configuration is defined in the SCSI-2 standard as the A cable. Older narrow (8-bit) SCSI adapters and external devices use a full-size Centronics-type connector that normally has wire latches on each side to secure the cable connector. Figure 8.4 shows what the lowdensity, 50-pin SCSI connector looks like Figure 8.4 Low-density, 50-pin SCSI connector The SCSI-2 revision added a high-density, 50-position, D-shell connector option for the A-cable connectors. This connector now is called Alternative 1. Figure 8.5 shows the 50-pin, high-density SCSI connector.

15 SCSI Cables and Connectors Chapter Figure 8.5 High-density, 50-pin SCSI connector The Alternative 2 Centronics latch-style connector remains unchanged from SCSI-1. A 68-conductor B-cable specification was added to the SCSI-2 standard to provide for 16- and 32-bit data transfers; the connector, however, had to be used in parallel with an A cable. The industry did not widely accept the B-cable option, which has been dropped from the SCSI-3 standard. To replace the ill-fated B cable, a new 68-conductor P cable was developed as part of the SCSI-3 specification. Shielded and unshielded high-density D-shell connectors are specified for both the A and P cables. The shielded high-density connectors use a squeeze-to-release latch rather than the wire latch used on the Centronics-style connectors. Active termination for single-ended buses is specified, providing a high level of signal integrity. Figure 8.6 shows the 68-pin, high-density SCSI connector Figure 8.6 High-density, 68-pin SCSI connector Drive arrays normally use special SCSI drives with what is called an 80-pin Alternative-4 connector, which is capable of Wide SCSI and also includes power signals. Drives with the 80-pin connector are normally hot-swappable they can be removed and installed with the power on in drive arrays. The 80-pin Alt-4 connector is shown in Figure 8.7. Pin 2 Pin 1 Figure 8.7 Pin pin Alt-4 SCSI connector. Apple and some other nonstandard implementations from other vendors used a 25-pin cable and connector for SCSI devices. They did this by eliminating most of the grounds from the cable, which unfortunately results in a noisy, error-prone connection. I don t recommend using 25-pin cables and connectors; you should avoid them if possible. The connector used in these cases was a standard female DB-25 connector, which looks exactly like a PC parallel port (printer) connector. Unfortunately, you can damage equipment by plugging printers into DB-25 SCSI connectors or by plugging SCSI devices into DB-25 printer connectors. So, if you use this type of SCSI connection, be sure it is marked well because there is no way to tell DB-25 SCSI from DB-25 parallel printer connectors by looking at them. The DB-25 connector is shown in Figure 8.8. Again, I recommend you avoid making SCSI connections using this type of cable or connector.

16 528 Chapter 8 The SCSI Interface 13 1 Figure 8.8 DB-25 SCSI connector SCSI Cable and Connector Pinouts The following section details the pinouts of the various SCSI cables and connectors. Two electrically different versions of SCSI exist: single-ended and differential. These two versions are electrically incompatible and must not be interconnected; otherwise, damage will result. Fortunately, very few differential SCSI applications are available in the PC industry, so you will rarely (if ever) encounter one. Within each electrical type (single-ended or differential), there are basically two SCSI cable types: A cable (Standard 8-bit SCSI) P cable (16-bit Wide SCSI) The 50-pin A-cable is used in most SCSI-1 and SCSI-2 installations and is the most common cable you will encounter. SCSI-2 Wide (16-bit) applications use a P cable instead, which has 68 pins. You can intermix standard and Wide SCSI devices on a single SCSI bus by interconnecting A and P cables with special adapters. SCSI-3 applications that are 32-bit wide would have used an additional Q cable, but this was finally dropped from the SCSI-3 standard after it was never implemented in actual products. SCSI cables are specially shielded with the most important high-speed signals carried in the center of the cable and less important, slower ones in two additional layers around the perimeter. A typical SCSI cable is constructed as shown in Figure 8.9. Outer Layer Data parity Media Layer Control signals PVC jacket Inner Layer REQ, ACK, Ground Shield Figure 8.9 Cross section of a typical SCSI cable. This specialized construction is what makes SCSI cables so expensive, as well as thicker than other types of cables. Note this specialized construction is necessary only for external SCSI cables. Cables used to connect devices inside a shielded enclosure (such as inside a PC) can use much less expensive ribbon cables.

17 SCSI Cable and Connector Pinouts Chapter The A cables can have pin-header type (internal) connectors or external shielded connectors, each with a different pinout. The P cables feature the same connector pinout on either internal or external cable connections. Single-Ended SCSI Cables and Connectors The single-ended electrical interface is the most popular type for PC systems. Tables 8.3 and 8.4 show all the possible single-ended cable and connector pinouts. The A cable is available in both internal unshielded and external shielded configurations. A hyphen preceding a signal name indicates the signal is Active Low. The RESERVED lines have continuity from one end of the SCSI bus to the other. In an A cable bus, the RESERVED lines should be left open in SCSI devices (but may be connected to ground) and are connected to ground in the bus terminator assemblies. In the P and Q cables, the RESERVED lines are left open in SCSI devices and in the bus terminator assemblies. Table 8.3 A-Cable (Single-Ended) Internal Unshielded Header Connector Signal Pin Pin Signal GROUND 1 2 -DB(0) GROUND 3 4 -DB(1) GROUND 5 6 -DB(2) GROUND 7 8 -DB(3) GROUND DB(4) GROUND DB(5) GROUND DB(6) GROUND DB(7) GROUND DB(Parity) GROUND GROUND GROUND GROUND RESERVED RESERVED Open TERMPWR RESERVED RESERVED GROUND GROUND GROUND ATN GROUND GROUND GROUND BSY GROUND ACK GROUND RST GROUND MSG GROUND SEL GROUND C/D GROUND REQ GROUND I/O Table 8.4 A-Cable (Single-Ended) External Shielded Connector Signal Pin Pin Signal GROUND DB(0) GROUND DB(1) GROUND DB(2) GROUND DB(3) GROUND DB(4) GROUND DB(5) GROUND DB(6) GROUND DB(7) GROUND DB(Parity) GROUND GROUND GROUND GROUND RESERVED RESERVED Open TERMPWR RESERVED RESERVED GROUND GROUND GROUND ATN GROUND GROUND GROUND BSY GROUND ACK GROUND RST GROUND MSG GROUND SEL GROUND C/D GROUND REQ GROUND I/O IBM used the SCSI interface in virtually all PS/2 systems introduced after These systems use a Micro-Channel SCSI adapter or have the SCSI Host Adapter built into the motherboard. In either case, IBM s SCSI interface uses a special 60-pin, mini-centronics type external shielded connector that is unique in the industry. A special IBM cable is required to adapt this connector to the standard 50-pin

18 530 Chapter 8 The SCSI Interface Centronics-style connector used on most external SCSI devices. The pinout of the IBM 60-pin, mini- Centronics style external shielded connector is shown in Table 8.5. Notice that although the pin arrangement is unique, the pin-number to signal designations correspond with the standard unshielded internal pin header type of SCSI connector. IBM has discontinued this design in all its systems because after the PS/2 series, all have used conventional SCSI connectors. The P cable (single-ended) and connectors are used in 16-bit Wide SCSI-2 applications (see Table 8.6 for the pinout). Table 8.5 IBM PS/2 SCSI External Signal Signal Name Pin Pin Name GROUND 1 60 Not Connected -DB(0) 2 59 Not Connected GROUND 3 58 Not Connected -DB(1) 4 57 Not Connected GROUND 5 56 Not Connected -DB(2) 6 55 Not Connected GROUND 7 54 Not Connected -DB(3) 8 53 Not Connected GROUND 9 52 Not Connected -DB(4) GROUND GROUND I/O -DB(5) GROUND GROUND REQ -DB(6) GROUND GROUND C/D -DB(7) GROUND GROUND SEL -DB(Parity) GROUND GROUND MSG GROUND GROUND GROUND RST GROUND GROUND RESERVED ACK RESERVED GROUND Open BSY TERMPWR GROUND RESERVED GROUND RESERVED GROUND GROUND ATN GROUND GROUND Table 8.6 P-Cable (Single-Ended) Internal or External Shielded Connector Signal Signal Name Pin Pin Name GROUND DB(12) GROUND DB(13) GROUND DB(14) GROUND DB(15) GROUND DB(Parity 1) GROUND DB(0) GROUND DB(1) GROUND DB(2) GROUND DB(3) GROUND DB(4) GROUND DB(5) GROUND DB(6) GROUND DB(7) GROUND DB(Parity 0) GROUND GROUND GROUND GROUND TERMPWR TERMPWR TERMPWR TERMPWR RESERVED RESERVED GROUND GROUND GROUND ATN GROUND GROUND GROUND BSY GROUND ACK GROUND RST GROUND MSG GROUND SEL GROUND C/D GROUND REQ GROUND I/O GROUND DB(8) GROUND DB(9) GROUND DB(10) GROUND DB(11)